This invention relates generally to radio navigation and positioning systems and, more specifically, to systems and methods for reconstructing continuous carrier wave signals despite gaps, discontinuities, and other variations in carrier signals.
Radio signals have been used as an aid to navigation and to obtain position estimates for decades. In much the same way that sailors could navigate near land using two or more light houses, the earliest systems used a directional antenna that determined a bearing to two or more radio transmitters. As long as line of sight could be maintained between the receiver and the two or more radio transmitters, a location of the receiver could be determined by triangulating the known locations of the two or more radio transmitters and the bearings to each of those radio transmitters. And although this approach may generally provide just a horizontal location (latitude and longitude) of the receiver, this may be adequate for localized navigation, such as the landing of aircraft or the navigation of ships around nearby navigational hazards.
Limiting radio navigation systems to local two-dimensional positioning, however, does not address many interesting positioning problems. For example, surveyors often desire to know the height/altitude of a location as well as its latitude and longitude, and pilots of aircraft often desire to know their altitude. To address these desires, more complex and longer distance radio navigation systems are typically utilized. Many of these radio navigation systems rely on the basic principle that radio waves generally propagate through the air at a known speed. By measuring the length of time it takes for a radio wave to propagate between a transmitter and a receiver, a distance between the transmitter and the receiver may be determined. By using the distance between the receiver and several transmitters with known locations, it is possible to determine the position of the transmitter by trilateration. For example, by using three transmitters, it is possible to determine the latitude, longitude, and altitude of the receiver. As additional transmitters are used and detected by the receiver, additional variables may be removed from the solution. For example, by adding time information to the radio signals, a fourth transmitter may be used to solve for the current time at the receiver.
Global Positioning Satellite (GPS) navigation, and more broadly Global Navigation Satellite System (GNSS) navigation, has become the standard for most military and civilian radio navigation applications. There exist in both military and civil sectors hundreds of millions of GPS or GNSS receivers that are used daily to provide real-time positioning and navigation. The GPS system is based on a constellation of approximately 24 to 32 middle-earth orbit (MEO) satellites that broadcast continuous carrier wave signals. A GPS receiver typically relies on the ability to receive signals from four or more satellites allowing the receiver to determine latitude, longitude, altitude, and time error at the receiver. For a typical GPS receiver, accuracy in location to within about 10 meters may be rapidly obtained.
A typical GPS receiver with an unobstructed view of the sky is generally able to receive signals from six or more satellites. However, local obstructions such as trees, terrain, buildings, and/or the like may often cause temporary or longer interruptions in the reception of the radio signals and/or make it difficult to receive the four signals used to determine the receiver position and time. To address these deficiencies in the GPS system, it may be possible to rely on signals from other transmitting sources, such as low earth orbit (LEO) satellites or ground-based transmitters. Unfortunately, these other signals may not be suitable for unaltered use with a conventional GPS receiver that is expecting to receive and possibly acquire and/or track a continuous carrier wave signal near a known or nominal frequency. Without these features, a GPS receiver may not be able to receive or process these other signals, which may result in a less accurate position and time solution or no solution availability at all.
Accordingly, it would be desirable to provide systems and methods for reconstructing continuous carrier wave signals at known frequencies for use with GPS receivers.